1. Field of the Invention
The present invention relates to a field-emission type cold cathode, and more particularly to an improvement in a field-emission type cold cathode to thereby make a divergence angle of emitted electron beams smaller and enhance symmetry about an axis along which electron beams are emitted.
2. Description of the Related Art
Field-emission type cold cathode have been developed as an electron source substituted for a hot cathode utilizing thermionic emission. A field-emission type cold cathode includes two electrodes one of which has a pointed end. Between the electrodes is generated a high electric field having intensity ranging from 2.times.10.sup.7 V/cm to 5.times.10.sup.7 V/cm or greater to thereby emit electrons into the atmosphere. Accordingly, the performance of a device including a field-emission type cold cathode is dependent on the sharpness of a pointed end. In general, it is necessary for a field-emission type cold cathode to have a pointed end having a radius of curvature equal to or smaller than several hundred angstroms. In addition, in order to generate an electric field, the two electrodes have to be spaced from each other by 1 .mu.m or smaller, and a voltage ranging from tens to hundreds of volts has to be applied across the two electrodes.
In practical use, thousands of to tens-of-thousand of the above mentioned cathodes are formed in a substrate in the form an array. Thus, field-emission type cold cathodes are generally manufactured by means of the technology for fabricating a semiconductor device having fine circuit patterns.
An example of the above mentioned field-emission type cold cathode has been suggested by C. A. Spindt: "A Thin-Film Field-Emission Cathode", J. Applied Physics, Vol. 39, No. 7, June 1968, pp. 3504-3505. According to Spindt, a pointed end of a field-emission type cathode is formed by depositing refractory metal such as molybdenum on an electrically conductive substrate.
FIG. 1 illustrates a structure of a field-emission type cathode suggested by Spindt. There is formed an insulating layer 34 on an electrically conductive substrate 35, and the insulating layer 34 is covered with an electrically conductive gate electrode 32. There is formed a hole 36 having a diameter of about 1 .mu.m, passing through the gate electrode 32 and the insulating layer 34 and terminating at the substrate 35. In the hole 36 is disposed an emitter electrode 31 so that the emitter electrode 31 is in electrical connection with the substrate 35 and further so that a pointed end of the emitter electrode 31 is located close to an inner edge of the gate electrode 32.
An anode electrode 11 is disposed facing the emitter electrode 31. The anode electrode 11 consists of a glass substrate 12, an electrically conductive, transparent film 13 formed on the glass substrate 12, and a thin fluorescent film 14 formed on the transparent film 13. A power supply 37 is disposed between the electrically conductive, transparent film 13 and the substrate 35, and also between the gate electrode 32 and the substrate 35.
By applying a positive voltage to both the transparent film 13 and the gate electrode 32 and a negative voltage to the substrate 35 and hence the emitter electrode 31, electron beams 15 are emitted from a pointed end of the emitter electrode 31. The illustrated cathode is called a vertical type field-emission cold cathode.
A field-emission type cathode as mentioned above may be used as an electron source for a planar display, a micro vacuum tube, electronic tubes such as a microwave tube and a cathode ray tube (CRT), and various sensors.
Electrons emitted from a pointed end of the emitter electrode 31 not only forward perpendicular to the substrate 35, but also extend in the vicinity of the emitter electrode 31 with a divergence half angle ranging from about 20.degree. to about 30.degree.. Thus, there is obtained a larger emission area than desired in a device having a fluorescent film to be excited by electron beams. When the illustrated field-emission type cathode is to be used as an electron source for an electronic tube, the extension of emission in the vicinity of the emitter electrode 31 would exert a degrading influence on beam-focusing carried out by electronic lenses.
One of solution to these problems is to provide an additional focusing electrode(s) to a device to thereby make a divergence angle smaller, as suggested by W. Dawson Kesling et al.: "Beam Focusing for Field-Emission Flat-Panel Displays", IEEE Transactions on Electron Devices, Vol. 42, No. 2, February 1995, pp. 340-347. This solution is grouped into two types.
One type includes, as illustrated in FIG. 2, a substrate 5 and a multi-layered structure having a first insulating layer 4, a gate electrode 2, a second insulating layer 4a and a focusing electrode 3 deposited on the substrate 5 in this order. There is formed a hole 36a passing through the focusing electrode 3, the second insulating layer 4a, the gate electrode 2 and the first insulating layer 4 and terminating at the substrate 5. In the hole 36a is disposed a conical emitter electrode 1 having a pointed end. By applying a lower voltage to the focusing electrode 3 than that of the gate electrode 2, electron beams are focused by an electric field generated by the focusing electrode 3. For instance, provided that the emitter electrode 1 is kept at 0 V, 70 V is applied to the gate electrode 2 and 10 V is applied to the focusing electrode 3.
The other type includes, as illustrated in FIG. 3, a substrate 5, an insulating layer 4 formed on the substrate 5, an annular-shaped gate electrode 2 formed on the insulating layer 4, and a focusing electrode 3 formed on the insulating layer 4 and at the same plane as the gate electrode 2 and surrounding the gate electrode 2. There is formed a hole 36b passing through the gate electrode 2 and the first insulating layer 4 and terminating at the substrate 5. In the hole 36b is disposed a conical emitter electrode 1 having a pointed end. By applying a lower voltage to the focusing electrode 3 than that of the emitter electrode 1, electron beams are focused by an electric field generated by the focusing electrode 3. For instance, provided that the emitter electrode 1 is kept at 0 V, 70 V is applied to the gate electrode 2 and -20 V is applied to the focusing electrode 3.
If the field-emission type cold cathode as illustrated in FIG. 2 is used, in order to make a divergence angle of electron beams emitted from the emitter electrode 1 smaller, an additional step is carried out for forming the second insulating layer 4a and the focusing electrode 3. In addition, since it is necessary to form a feeder line extending from the gate electrode 2 and the focusing electrode 3 (for supplying power thereto) with the feeder line being electrically insulated therefrom, it is not avoidable to involve complex photolithography and etching steps for the formation of wiring pattern.
If the hole 36a in which the emitter electrode 1 is disposed is formed by single photolithography, it is necessary to provide an etching depth almost twice as long compared with an etching depth for a field-emission type cathode having no focusing electrodes. Thus, it is difficult in dry etching to properly determine a selection ratio of an etching depth to a photoresist layer (acting as a mask). In addition, it is difficult in wet etching to control side etching. If the focusing electrode 3 and the gate electrode 2 are to be separately patterned by carrying out photolithography twice, the above mentioned problems can be overcome. However, another problem arises that it is quite difficult or almost impossible to eliminate misregistration between the two patterns, and thus, such misregistration would exert a degrading influence on the symmetry of focusing effects.
In the field-emission type cold cathode as illustrated in FIG. 3, the focusing electrode 3 can be formed by patterning simultaneously with the gate electrode 2. Accordingly, the problems which occur in the cathode illustrated in FIG. 2 do not arise.
However, the field-emission type cold cathode illustrated in FIG. 3 has another problem. FIG. 4A illustrates a field-emission type cold cathode having been suggested in Japanese Unexamined Patent Publication No. 7-14501. The illustrated cold cathode is comprised of a circular-shaped gate electrode 2, a plurality of emitter electrodes 1 each disposed in a hole formed with the gate electrode 2, an annular-shaped focusing electrode 3 surrounding the gate electrode 2, and two focusing pads 7, 8 formed outside the gate electrode 2 and the focusing electrode 3. As illustrated, the gate electrode 2 is connected to the gate pad 7 through a feeder line 9, and the focusing pad 8 is connected to the focusing electrode 3 through a feeder line 9a.
In the illustrated cold cathode of FIG. 4A, if the feeder line 9 is to be formed at the same plane as the gate electrode 2 and the focusing electrode 3, the focusing electrode 3 is partially cut in order to draw the feeder line 9 from the gate electrode 2 (located inside the focusing electrode 3) to the gate pad 7 located outside the focusing electrode 3. By partially cutting the focusing electrode 3, the focusing electrode 3 is no longer present in a direction in which the feeder line 9 extends. Hence, asymmetry would arise in an electric field generated by a voltage applied to the focusing electrode 3, resulting in electrons being attracted to the voltage of the feeder line 9. As a result, electron beams spread towards the direction in which the feeder line 9 extends.
For instance, in such a device as illustrated in FIG. 1 where the fluorescent film 14 formed over the anode electrode 11 is to be excited by the electron beams 15, emission area 27 of the device spreads in a direction in which the focusing electrode 3 is partially cut, as illustrated in FIG. 4B. Namely, if the devices illustrated in FIG. 4A are arranged in a plane and used as a display device, there would arise problems that resolution is deteriorated and axis-symmetry of beams is also deteriorated when used as an electron beam source.
One of methods of drawing the feeder line 9 from the gate electrode 2 through the focusing electrode 3 in a device including the annular-shaped focusing electrode 3 surrounding the gate electrode 2, as illustrated in FIG. 3, is suggested by Christophe Py et al., "Microtip electron Beam Refocusing by Surrounding Ring", 42nd Applied Physics Related-Association Conference, 1995, 30p-T-5. FIG. 5A illustrates the suggested structure including a gate electrode 2, an emitter electrode 1 disposed in a hole formed with the gate electrode 2, a focusing electrode 3 almost surrounding the gate electrode 2, gate pads 7 located outside the focusing electrode 3, and a pair of feeder lines 9 extending from the gate electrode 2 to the gate pads 7. As illustrated, the feeder lines 9 extend symmetrically about the gate electrode 2 in left-right directions in which the focusing electrode 3 is partially cut.
FIG. 5B illustrates emission area 28 established on the fluorescent film 14 formed on the anode electrode 11 (see FIG. 1) by electron beams emitted from the emitter electrode 1 illustrated in FIG. 5A. Electron beams emitted from the emitter electrode 1 illustrated in FIG. 5A have a plane-symmetrical cross-section. However, the emission area 28 spreads along the directions in which the focusing electrode 3 is partially cut, and hence the focusing performance of the focusing electrode 3 is not improved.
As an alternative, as illustrated in FIG. 6, the focusing electrode 3 may be partially three-dimensionally formed above the feeder line 9 with an insulating layer 24 being filled between the focusing electrode 3 and the feeder line 9, to thereby avoid the focusing electrode 3 from being partially cut. However, there has to be carried out the increased number of steps of formation of the insulating layer 24 and of patterning, which would eliminate an advantage of avoiding the focusing electrode 3 from being partially cut.
As mentioned above, though the cathode illustrated in FIG. 3 having the annular-shaped focusing electrode 3 at the same plane as the gate electrode 2 provides an advantage of making it possible to simplify the fabrication process, it does have a problem that the feeder line 9 extending from the gate electrode 2 partially cuts the focusing electrode 2, resulting in that the performance of focusing electron beams is deteriorated and axis-symmetry of electron beams is also deteriorated.